A. Description of the Related Art
Ultrasound imaging systems use time delays and/or phase rotation means to form focused ultrasound beams. On transmit, time delays and/or phase rotation means are used to bring ultrasound pulses from different transducer elements to the desired focal point with temporal alignment and phase coherence. Likewise, on receive, time delays and/or phase rotation means are used to bring reflected ultrasound pulses arriving at different transducer elements from the desired focal points into temporal alignment and phase coherence. The time delays and phases used to focus the ultrasound beam are specified assuming a constant propagation velocity (nominally 1540 m/s in human soft tissue) in the medium through which ultrasound pulses propagate.
However, human soft tissue is not homogenous; it is composed of regions of acoustically differing tissues, such as fat, muscle and blood, in which the local propagation velocity varies. The path dependent speed of sound in tissue distorts the transmitted and reflected wavefronts propagating through the tissues by introducing delay variations from the nominal. These delay variations degrade the quality of focus, thus reducing the spatial resolution and contrast resolution seen in the image.
B. Patents and Literature
By way of example, the following United States patents and literature, all of which are incorporated by reference herein, discuss various aspects of ultrasound imaging. The patents and literature include:
______________________________________ U.S. Pat. No.: Title: Inventor(s): ______________________________________ 4,471,785 ULTRASONIC IMAGING David A. Wilson SYSTEM WITH James L. Buxton CORRECTION FOR Philip S. Green VELOCITY IN- Donald J. Burch HOMOGENEITY AND John Holzener MULTIPATH INTER- S. David Ramsey, Jr. FERENCE USING AN ULTRASONIC IMAGING ARRAY 4,817,614 METHOD AND Dietrich Hassler APPARATUS FOR Heinz Eschenbacher ADAPTIVE FOCUSING Wolfgang Haerer IN A MEDICAL ULTRASOUND IMAGING APPARATUS 4,835,689 ADAPTIVE COHERENT Matthew O'Donnell ENERGY BEAM FORMATION USING PHASE CONJUGATION 4,852,577 HIGH SPEED ADAPTIVE Stephen W. Smith ULTRASONIC PHASED Gregg E. Trahey ARRAY IMAGING SYSTEM 4,937,775 APPARATUS FOR THE William E. Engeler THE CROSS- Matthew O'Donnell CORRELATION OF A PAIR OF COMPLEX SAMPLED SIGNALS 4,989,143 ADAPTIVE COHERENT Matthew O'Donnell ENERGY BEAM Stephen W. Flax FORMATION USING ITERATIVE PHASE CONJUGATION 5,113,866 METHOD FOR Dietrich Hassler ULTRASOUND IMAGING Klaus Killig 5,172,343 ABERRATION Matthew O'Donnell CORRECTION USING BEAM DATA FROM A PHASED ARRAY ULTRASONIC SCANNER ______________________________________
2. Literature
a. M. Hirama, et al., "Adaptive Ultrasonic Array Imaging System Through an Inhomogeneous Layer," Journal of the Acoustical Society of America, Vol. 71, pp. 100-109, January 1982. PA1 b. T. Yokota, et al., "Active Incoherent Ultrasonic Imaging Through an Inhomogeneous Layer," Journal of the Acoustical Society of America, Vol. 77, pp. 144-152, January 1985. PA1 c. S. Flax, et al., "Phase Aberration Correction Using Signals From Point Reflectors and Diffuse Scatterers," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 35, pp. 758-774, November 1988. PA1 d. L. Nock, et al., "Phase Aberration Correction In Medical Ultrasound Using Speckle Brightness As a Quality Factor," Journal of the Acoustical Society of America, Vol. 85, pp. 1819-1833, May 1989. PA1 e. M. O'Donnell, et al., "Correlation-Based Aberration Correction in the Presence of Inoperative Elements," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, pp. 700-707, November 1992.
All of the above disclose systems for determining aberration corrections using special adaptive modes nonconcurrent with imaging which may be used to correct nominal focusing delay and phase values during transmit beamformation and/or receive beamformation for the defocusing effects caused by aberrating regions. U.S. Pat. Nos. 4,471,785, 4,817,614 and 4,852,577 show means to determine focusing corrections at a single depth and to apply the aberration correction values obtained during the adaptive mode to all focal points during the imaging modes.
However, because optimal aberration corrections vary as the focus is dynamically varied in depth during receive beamformation, corrections determined at a single depth (or a few depths) do not optimally correct focus at all depths. On the other hand, determining aberration corrections at many depths through direct measurement, as suggested by U.S. Pat. Nos. 4,835,689, 4,937,775, 4,989,143 and 5,172,343, may require undesirable increases in 1) processing power, 2) computation time (which may slow frame rate), 3) memory, and 4) the number of non-imaging scan lines (which further slows frame rate).
None of the related art is able to take aberration correction values obtained from one range, scan mode, geometry, and transmit frequency and apply them to alternative ranges, alternative scan modes, alternative scan geometries, and/or alternative transmit/receive frequencies. For example, if an imaging system were to acquire both color Doppler flow scan lines (color Doppler F-mode) using a steered linear scan geometry and gray scale image scan lines (B-mode) using a Vector.RTM. scan geometry, the related art systems would require separate aberration correction values for each mode, geometry, and frequency and would not be able to use aberration correction values obtained from one mode, geometry, and frequency to apply to the other modes, geometries, and frequencies.
Accordingly, it is desired to provide a method and apparatus for determining aberration correction values that can be applied for all focal points for any scan mode, scan geometry or frequency without reducing frame rate or requiring special or separate acquisition modes apart from normal imaging modes.